Disk-type motors and generators have some important advantages over standard cylindrical machines. These advantages include higher power density and lower noise levels. Ordinary cylindrical-type machines are inherently noisy due to radial flexure of the machine housing. In contrast, disk-type machines are inherently quieter because the housing flexure resulting from the attraction and repulsion forces between the stator and rotor poles is axial. As a result, the ends of the housing of a disk-type machine vibrate due to attraction and repulsion forces, but the cylindrical portion of the housing is much stiffer in the axial direction than in the radial direction so vibration is reduced. In addition it is much easier to isolate axial vibrations from the supporting structure than it is to isolate radial vibrations. In applications where silent electrical machine operation is necessary, such as in submarines, the disk-type motor is particularly advantageous.
However, disk-type designs in the past have suffered from lower efficiency, high reactance, and manufacturing difficulties. Manufacturing complexity results from the fact that stator teeth in prior art designs are laminated in the axial and peripheral directions but are tapered in the radial direction. Thus, each radial layer of punchings must be a slightly different size than the previous layer. In addition, the material between stator teeth that supports the teeth must be of a different material so as to be non-magnetic, non-conducting, and light in weight for maximum efficiency and power density. Accurately pinning all of these separate laminations together to obtain a structurally sound stator assembly is also difficult. The same problems occur in constructing a rotor for such machines.
Disk-type machines with one stator and one rotor have been made in small sizes, as described in U.S. Pat. No. 4,757,222 to Shiraki et al. However, axially directed magnetic forces become prohibitively large in machines of power greater than 100 kilowatts.
These forces can be cancelled in a machine that has at least two axial air gaps, as shown in U.S. Pat. Nos. 4,237,396 to Blenkinsop et al. and 4,288,709 to Matthias et al. This concept is also suggested in the article, "On the Feasibility and Advantages of Disk-Type Electric Machines," by A.S. Kurbasov appearing in Electric Technology U.S.S.R., No. 1, at pages 54-62. A prior art machine of this type is shown in FIG. 1. As shown in FIG. 1, these machines employ two or more stator coils 108. The machine of FIG. 1 includes rotor coils 102 mounted on field poles 103 which are in turn mounted on rotor supports 104 rotating with shaft 106. Stator coils 108 are mounted on stator support disk 110 which is fixed to stator frame 112. Two axial air gaps 114 are defined between stator coils 108 and rotor coils 102. As shown in FIG. 2, such machines may use a composite stator disk 202 with laminated iron teeth 204 arranged to form parallel sided slots 206 therebetween. The disadvantage of these prior art techniques is that the multiple stator windings increase the machine reactance and decrease efficiency, primarily because of the extra sets of stators and the increase in coil end turns required.
Therefore, there is a need for an improved disk-type machine that offers greater efficiency, has lower reactance, and is less expensive and less difficult to manufacture than prior-art disk-type machines.